Bonding method and bonding apparatus

ABSTRACT

A bonding method and a bonding apparatus whereby the uniformity of an adhesive layer during application is ensured, variation of the adhesive layer during manufacture can be suppressed and the occurrence of air bubbles can be reduced. The bonding apparatus includes: a first spin coating device 1 for applying an adhesive B1 onto one surface of a first substrate P 1;  a second spin coating device for applying an adhesive B 2  onto one surface of a second substrate P 2,  more thinly than the adhesive B 1  on the first substrate; a pre-irradiation unit  4  for provisionally curing the adhesive B1 on the first substrate P 1;  a bonding unit  5  for bonding together the surface of the first substrate P 1  to which the adhesive B1 has been applied and the surface of the second substrate P 2  to which the adhesive B 2  has been applied; and a post-irradiation unit 6 which cures the adhesive B 1  and B 2  between the first substrate P 1  and the second substrate P 2.

TECHNICAL FIELD

The present invention relates to a bonding method and bonding apparatuswhich improve the application of an adhesive onto a substrate beforebonding, in order to manufacture a recording medium in which a pair ofsubstrates are bonded together by means of an adhesive.

BACKGROUND ART

Currently, in the field of optically readable recording media such asoptical disks or magneto-optical disks, disks having a wide variety ofdifferent specifications are used, for instance, there are read-onlymedia and media which enable rewriting of the recorded information. Anoptical disk such as a DVD, for example, is basically manufactured byproviding information recording regions on one side or both sides of twosubstrates, and bonding the substrates together by means of an adhesive.The adhesive layer for bonding the substrates together is required tohave an extremely accurate thickness in order that information can beread and written accurately by laser.

One example of a procedure for manufacturing a bonded disk of this kindis described now with reference to FIG. 6. Firstly, two polycarbonatesubstrates P are previously formed by extrusion molding, and a metalfilm for reflecting laser light (recording film) is formed by sputteringin a sputtering chamber. As shown in FIG. 6A, an ultraviolet-curableadhesive is applied to the bonding surfaces of two substrates P, and theadhesive is caused to spread by spin coating. Spin coating is a processin which an adhesive is applied dripwise (dropt) by an applicationdevice K about the perimeter of the center of a substrate P and thesubstrate P is spun at high speed, whereby a thin film of adhesive(adhesive layer R) is formed on the substrate P, and surplus adhesive isscattered.

As shown in FIG. 6B, one of the pair of substrates P on which anadhesive layer R has been formed in this way is held by a chuck E of acenter pin G and the other of the substrates is mounted on the mountingsurface F of a turntable or susceptor, in such a manner that therespective adhesive layers R are facing each other in parallel,whereupon the substrates are introduced into the lower part of a vacuumchamber C. Thereupon, as shown in FIG. 6C, a vacuum chamber S is formedby lowering and sealing the vacuum chamber C, and the pressure of theperiphery of the substrates P is reduced from atmospheric pressure to avacuum by evacuating air from the vacuum chamber S by means of an airevacuation device.

The chuck E holding one substrate P is closed inside the space that hasbeen reduced to vacuum pressure, and the substrates P are lowered, inaddition to which a pressing section T is lowered and caused to applypressure by a drive source, such as a cylinder, thereby bonding the onesubstrate P to the other substrate P. A vacuum pressure is createdduring bonding in order to eliminate as far as possible any gasmolecules present between the surfaces that are to be bonded.

Thereupon, as shown in FIG. 6D, the peripheral atmosphere of thesubstrates P that have been bonded together is either returned toatmospheric pressure or raised to a pressure higher than atmosphericpressure and then returned to atmospheric pressure, by introducing air.By returning the substrates to atmospheric pressure in this way, any airbubbles remaining in the adhesive layers R are caused to contractgradually to the pressure difference with respect to the vacuum. Afterthe substrates P have been left at atmospheric pressure for severalseconds to several tens of seconds until the air bubbles have beencontracted sufficiently, then as shown in FIG. 6E, the adhesive layer Ris cured by irradiating ultraviolet light onto the whole of thesubstrates by a light source U. By this means, the two substrates P arebonded strongly together, thereby completing a disk.

In a disk which is manufactured by bonding together substrates ontowhich an adhesive has been applied as described above, when laser lightused for reading and writing information is irradiated onto the disk,then it is necessary for the adherence of dirt and the occurrence of airbubbles to be reduced as far as possible in such a manner that a stablespot is formed. In order to achieve this, in the prior art, bonding iscarried out in a vacuum and pressure is applied after bonding. Onetechnique for applying pressure in this way is to use atmosphericpressure and expose the disk to an air atmosphere for a prescribedperiod of time (air resting) (see Patent Reference 1).

Patent Reference 1: Japanese Patent Application Publication No.2006-48855.

However, in a disk of this kind, in order to form a stable laser spot,it is also necessary for the disk to be a flat disk which is free ofwarping or distortion. Consequently, in a disk of this kind, it isdesirable that the film thickness of the adhesive layer during bondingshould be as uniform as possible. For example, it is known that thevariation (fluctuation) in the circumferential direction(in-circumference variation) of the film thickness of an adhesive layerapplied by spin coating becomes greater from the inner circumference ofthe disk toward the outer circumference.

With increase in the recording density in recent years, as in the caseof HD (High Definition DVD) and BD (Blu-ray Disc), the uniformity of theadhesive layer required in the final disk has become subject toextremely strict requirements. For example, in a conventional DVD, therange of variation is approximately 30 μm, but in HD or BD disks,greater accuracy of approximately 10 μm is necessary. Furthermore, withthis increase in recording density, in order to prevent reading errors,it has become necessary to reduce yet further the size and quantity ofadhering dirt and air bubbles.

However, as described in the abovementioned Patent Document 1 if thesubstrates are left in an air atmosphere after bonding in order toreduce air bubbles, then the in-circumference variation of the adhesivelayer increases. This is thought to be because the time from thedeposition of the adhesive until its curing becomes long and hence thereis increased chance of the adhesive flowing during the curing process.For example, FIG. 7 shows individual and average measurement results forthe rate of in-circumference variation in a case where the substratesare not left to rest after bonding together, and in a case where thesubstrates are left for 20 seconds after bonding together. As shown inFIG. 7, it can be seen that when the substrates are left to rest, thereis overall deterioration in the in-circumference variation and there issignificant variation in the outer circumference portion.

DISCLOSURE OF THE INVENTION

The present invention was devised in order to resolve the aforementionedproblems of the prior art, an object thereof being to provide a bondingmethod and a bonding apparatus whereby uniformity of the adhesive layerduring application can be ensured, while suppressing variation in theadhesive layer during manufacture.

In order to achieve the object described above, the present invention isa bonding method for bonding a first substrate and a second substrate bymeans of an adhesive which undergoes curing by irradiation ofelectromagnetic radiation, characterized in comprising: applying theadhesive, at respectively different thicknesses, to one surface of thefirst substrate and one surface of the second substrate; irradiatingelectromagnetic radiation onto the thicker of the adhesive applied tothe first substrate and the adhesive applied to the second substrate;bonding together the surface of the first substrate to which theadhesive has been applied and the surface of the second substrate towhich the adhesive has been applied; and irradiating electromagneticradiation onto the adhesive between the first substrate and the secondsubstrate.

Another mode of the present invention is a bonding apparatus for bondingtogether a first substrate and a second substrate by means of anadhesive which undergoes curing by irradiation of electromagneticradiation, characterized in comprising: at least one application unitfor applying the adhesive, at respectively different thicknesses, to onesurface of the first substrate and one surface of the second substrate;a pre-irradiation unit for irradiating electromagnetic radiation ontothe thicker of the adhesive applied to the first substrate and theadhesive applied to the second substrate; a bonding unit for bondingtogether the surface of the first substrate to which the adhesive hasbeen applied and the surface of the second substrate to which theadhesive has been applied; and a post-irradiation unit for irradiatingelectromagnetic radiation onto the adhesive between the first substrateand the second substrate.

In the inventions described above, since the thicknesses of the adhesiveto be applied to the substrates are made different before bondingtogether the first substrate and the second substrate andelectromagnetic radiation is irradiated onto the thicker of theadhesives, then the thicker region is provisionally cured andin-circumference variation is suppressed, in addition to which theoccurrence of air bubbles after bonding can be reduced.

Another mode of the present invention is a bonding method for bonding afirst substrate and a second substrate by means of an adhesive whichundergoes curing by irradiation of electromagnetic radiation,characterized in comprising: applying the adhesive, at respectivelydifferent thicknesses, to one surface of the first substrate and onesurface of the second substrate; irradiating electromagnetic radiationonto both the adhesive applied to the first substrate and the adhesiveapplied to the second substrate; bonding together the surface of thefirst substrate to which the adhesive has been applied and the surfaceof the second substrate to which the adhesive has been applied; andirradiating electromagnetic radiation onto the adhesive between thefirst substrate and the second substrate.

A further mode of the present invention is a bonding apparatus forbonding together a first substrate and a second substrate by means of anadhesive which undergoes curing by irradiation of electromagneticradiation, characterized in comprising: at least one application unitfor applying the adhesive at respectively different thicknesses to onesurface of the first substrate and one surface of the second substrate;a pre-irradiation unit for irradiating electromagnetic radiation ontoboth the adhesive applied to the first substrate and the adhesiveapplied to the second substrate; a bonding unit for bonding together thesurface of the first substrate to which the adhesive has been appliedand the surface of the second substrate to which the adhesive has beenapplied; and a post-irradiation unit for irradiating electromagneticradiation onto the adhesive between the first substrate and the secondsubstrate.

In modes such as those described above, since electromagnetic radiationis irradiated onto both of the adhesives before bonding together thefirst substrate and the second substrate, it is possible to carry outbonding in a state where the flowing movement of both adhesives has beencontrolled, and therefore it is possible to suppress in-circumferencevariation in the adhesive layer, as well as being able to reduce theoccurrence of air bubbles after bonding.

In a further mode of the invention, the application unit has at leastone spin coating device for spreading the adhesive by causing the firstsubstrate and the second substrate to rotate, control means beingfurther provided for controlling the spin coating device such thatconditions of rotation are respectively different for the firstsubstrate and the second substrate is also provided.

In modes such as those described above, it is possible to control theapplication thickness of the adhesive by changing the conditions ofrotation, such as the rotational speed.

In a further mode of the invention, the application unit comprises atleast one spin coating device for spreading the adhesive by rotating thefirst substrate and the second substrate, and control means beingfurther provided for controlling the spin coating device such that thenumber of spreading operations is respectively different for the firstsubstrate and the second substrate is also provided.

In modes such as those described above, the application thickness of theadhesive is controlled on the basis of the number of superimposedapplication operations performed, by changing the number of adhesivespreading operations, and therefore uniformity can be ensured readilyeven if the adhesive is formed thickly.

As described above, according to the present invention, it is possibleto provide a bonding method and a bonding apparatus whereby theuniformity of an adhesive layer during application can be ensured,variation in the adhesive layer during manufacture can be suppressed,and the occurrence of air bubbles can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory diagram showing the composition of oneembodiment of a bonding apparatus according to the present invention;

FIG. 2 is a simplified vertical cross-section showing a first spincoating device according to the embodiment in FIG. 1;

FIG. 3 is a flowchart showing the processing sequence of the embodimentin FIG. 1;

FIG. 4 is an explanatory diagram showing the level of occurrence of airbubbles in disks having different film thickness ratios which aremanufactured in accordance with an embodiment of the present invention;

FIG. 5 is an explanatory diagram showing one embodiment of a case wheretwo pre-irradiation units are disposed in the bonding apparatusaccording to the present invention;

FIGS. 6A to 6E are explanatory diagrams showing a substrate bondingprocedure according to the prior art, in which FIG. 6A shows a step ofspreading adhesive, FIG. 6B shows a step of introducing into a vacuumchamber, FIG. 6C shows a bonding step, FIG. 6D shows an air restingstep, and FIG. 6E shows an adhesive curing step; and

FIG. 7 is an explanatory diagram showing the in-circumferencedistribution of the adhesive layer thickness of a disk manufactured inaccordance with prior art technology.

BEST MODE FOR CARRYING OUT THE INVENTION

Next, a preferred embodiment of the present invention (hereinafter,called “embodiment”) will be described with reference to the drawings.In the present embodiment, adhesive is applied to different thicknesseson a pair of substrates, and the adhesive is provisionally cured,whereupon the substrates are bonded together, and by this meansin-circumference variation of the adhesive is suppressed.

Composition of the Embodiment

Firstly, the composition of the bonding apparatus according to thepresent embodiment (hereinafter, called the “present apparatus”) will bedescribed with reference to FIG. 1 and FIG. 2. The present apparatusconstitutes a portion of a disk manufacturing apparatus, and thesubstrate molding device and metal film forming device disposed to theupstream process side of the present apparatus and the mechanisms fortransferring the substrates between the respective devices use commonlyknown technology and therefore description thereof is omitted here.

The present apparatus comprises a first spin coating device 1, a secondspin coating device 2, a pre-irradiation unit 4 constituted by aturntable 3, a bonding unit 5, a post-irradiation unit 6, and the like.The first spin coating device 1 is a device which coats anultraviolet-curable adhesive B1 by spin coating onto one substrate P1that is to be bonded. This first spin coating device 1 comprises aturntable 11 on which a substrate P1 is mounted and a drive source 12which causes the turntable 11 to rotate, and it serves to cause adhesiveB1 that has been dripped onto the substrate by an adhesive supply unit(not illustrated) to spread due to the rotation of the substrate P1.

Furthermore, as shown in FIG. 2, the first spin coating device 1comprises an irradiation device 13 which irradiates ultraviolet light(UV) onto the adhesive B1 on a substrate P1, and a heating device 14which applies heat. The irradiation device 13 is a device whichirradiates ultraviolet light in the form of spots about the periphery ofthe central hole of the substrate P1, and is composed in such a mannerthat ultraviolet light from a light source is guided along an opticalfiber. It may also be composed in such a manner that the irradiationintensity can be adjusted by using ultraviolet light LEDs for the lightsource. The heating device 14 is a device for heating the substrate P1in the vicinity of its outer circumference. The heating device 14 mayemploy an infrared (IR) irradiation unit or a heater, for instance.

The second spin coating device 2 is a device which coats anultraviolet-curable adhesive by spin coating onto another substrate P2that is to be bonded, as shown in FIG. 1. This second spin coatingdevice 2 comprises a turntable 21 on which a substrate P2 is mounted anda drive source 22 which causes the turntable 21 to rotate, and it servesto cause adhesive B2 that has been dripped onto the substrate by anadhesive supply unit (not illustrated) to spread due to the rotation ofthe substrate P2.

The turntable 3 has a first introduction position 31, corresponding tothe pre-irradiation unit 4, where the substrate P1 is introduced, asecond introduction position 32 where the substrate P2 is introducedafter being inverted by an inverting device (not illustrated) in such amanner that the bonding surfaces are facing each other, a bondingposition 33 which corresponds to the bonding unit 5, an ultravioletlight irradiation position 34 which corresponds to the post-irradiationunit 6, and an output position 35 where the completed disk D is outputto the next stage. This turntable 3 is composed so as to turnintermittently in accordance with the respective positions describedabove, by means of a drive mechanism (not illustrated).

The pre-irradiation unit 4 is a device which performs provisional curingby irradiating ultraviolet light by a UV irradiation device in an airatmosphere onto the adhesive B1 which has been applied to the substrateP1. Here, “in an air atmosphere” means in an environment which inhibitscuring, for example, an oxygen-containing gas atmosphere. In general, itis easiest to use normal air, but any environment which contains oxygenor which inhibits curing would be suitable.

The bonding unit 5 is a device which bonds together the substrates P1and P2 in a vacuum. The bonding unit 5 has a vacuum chamber which isoperated by an elevator mechanism, a vacuum source which reduces theinterior of the vacuum chamber to a vacuum, and a pressing unit which isoperated by an elevator mechanism and applies pressure to the substratesP1 and P2, but since this involves commonly known technology, it is notdescribed further here.

The post-irradiation unit 6 is a device which irradiates ultravioletlight in a vacuum onto the bonded substrates P1 and P2, by means of theUV irradiation device, and thereby fully cures the adhesives B1 and B2between the substrates P1 and P2. The post-irradiation unit 6 alsocomprises a vacuum chamber which is operated by an elevator mechanismand a vacuum source which reduces the interior of the vacuum chamber toa vacuum pressure, and the like, but since this involves commonly knowntechnology, it is not described further here.

Irradiation is carried out in a vacuum in order to remove any factorswhich may inhibit curing, such as the presence of oxygen, or the like,but it is not absolutely necessary to carry out irradiation in anenvironment which is free of oxygen. This is because the adhesivebonding surfaces of the substrates P1 and P2 after they have been bondedtogether form a substantially unified body, which is cured regardless ofthe atmosphere. If bonding is carried out in an air atmosphere, then theend face on the outer circumference comes into contact with the air, butthis portion reaches full curing in the course of storage (over severaldays) on the manufacturing line. If irradiation is carried out in avacuum as described above, then a merit is obtained in that the end faceon the outer circumference can be cured more reliably. A similarbeneficial effect can be obtained by removing (purging) oxygen with aninert gas (N₂).

The supply volume of adhesive from the adhesive supply unit, therotation of the turntable and speed of rotation of same, the emission oflight by the irradiation devices, and the operation of the heatingdevice, the elevator mechanism and the vacuum source, and the like, arecontrolled by means of a control device. This control device may berealized, for example, by a dedicated electronic circuit or a computerwhich is operated by a prescribed program. Therefore, one mode of thepresent invention is a computer program for controlling the operation ofthe present apparatus according to the sequence described below, or arecording medium which stores such a computer program.

Action of the Embodiment

The substrate bonding procedure carried out by the present apparatusdescribed above will now be explained with reference to FIG. 1 and FIG.2, and the flowchart in FIG. 3. In preceding steps, a semi-transparentreflective film is formed by sputtering on one substrate P1 and a fullyreflective metal film is formed by sputtering on the other substrate P2.

For the Substrate P1, in the first spin coating device 1, as shown inFIG. 1, an ultraviolet-curable adhesive is applied dripwise onto theperiphery of the central hole, and the adhesive is caused to spread byrotating the turntable 11 at high speed (step 301). For example, anadhesive having a viscosity of 430 mPas is used, the applicationpressure is 0.2 MPa, the application time is 0.15 sec. This adhesive isdistributed for 1 sec by high-speed spinning at 10,000 rpm.

Thereupon, as shown in FIG. 2, the substrate P1 is rotated in such amanner that the adhesive does not perform a flowing movement (forexample, at a speed of 120 rpm to 300 rpm), and ultraviolet light isirradiated in the form of spots at the periphery of the central hole bythe irradiation device 13. Consequently, a portion which is cured in aring shape (cured portion) is formed in the adhesive that has beenspread (step 302). In this case, the portion where the ultraviolet lightintensity is strong is cured fully, but as the position moves toward theouter circumference, an inhibiting effect of the oxygen occurs on theadhesive, the surface of the adhesive does not solidify while theinterior does solidify, and the interior is progressively cured to alesser extent toward the outer circumference.

Next, an ultraviolet-curable adhesive is applied dripwise again on topof the adhesive of the substrate P1 on which the cured portion has beenformed, and by rotating the turntable 11 at high speed, the adhesive iscaused to spread (step 303). For instance, using the same adhesive asthat used in the first application, the application pressure is 0.2 MPa,the application time is 0.6 sec, and the adhesive is spread for 1 sec byhigh-speed spinning at 4000 rpm. In this case, the adhesive is spread byapplying heat locally using the heating device 14. For instance, a spotheater is used as the heat source, the wavelength is 700 to 3000 nm, theoutput setting is 350 W, the heating range is 40 mm to 60 mm in theradial direction of the substrate P1, and the heating time is 1 sec.

By applying the adhesive to the substrate P1 in this way, the adhesivewhich is warmed, thereby reducing the viscosity of the adhesive, isdistributed and spread readily to the outer circumference as a result ofthe centrifugal force created by the rotation of the substrate.Alternatively, the adhesive receives thermal energy, and thevolatilization volume is raised. Therefore, the adhesive remaining atthe outer circumference becomes thinner, thus suppressing increase inthe thickness of the adhesive, and therefore it is possible to achieve auniform thickness throughout the substrate. It is also possible to applya heated air flow to the outer circumference simultaneously, so as tosupplement the heating of the adhesive.

Thereafter, as shown in FIG. 1, the substrate P1 is introduced onto theturntable 3 in such a manner that the surface of the applied adhesive B1is facing upwards, as described above (step 304). In the pre-irradiationunit 4, ultraviolet light is irradiated onto the whole surface of thesubstrate in an air atmosphere by the UV irradiation device, and theadhesive B1 is provisionally cured to a degree whereby the applied formof the adhesive is not disturbed (step 305). For instance, compared toconditions used for normal full curing (50 mW/cm^(2×5)s), light isirradiated for approximately one half of the irradiation time (2s) atthe same intensity.

With a general ultraviolet-curable resin (adhesive), if ultravioletlight is irradiated onto the whole surface in an air atmosphere, thenfull curing does not occur at a normal irradiation intensity. This isbecause the presence of air in the vicinity of the surface of the resininhibits the curing process. In other words, when ultraviolet light isirradiated in an air atmosphere, it is possible to perform provisionalcuring while maintaining the adhesive on the surface. For example, ithas been demonstrated that full curing is not carried out even underconditions of 1000 mW at 1 to 2 seconds. However, the irradiationconditions during provisional curing are not limited to those describedabove.

Furthermore, as shown in FIG. 1, adhesive is applied to the othersubstrate P2 by the second spin coating device 2 (step 306). Forinstance, the application pressure is 0.2 MPa, the application time is0.6 sec, and the adhesive is distributed for 1 sec by high-speedspinning at 6,000 rpm. As described above, the adhesive B1 on thesubstrate P1 is made thicker and the adhesive B2 on the substrate P2 ismade thinner, in accordance with the rotating conditions set for spincoating and the number of application operations. Thereupon, thesubstrate P2 is inverted by the inverting device (step 307), andintroduced over the substrate P1 in such a manner that the applicationsurface of the adhesive B2 is facing downwards (step 308).

Thereupon, the two substrates P1 and P2 are conveyed to the bonding unit5, and bonding is carried out in a vacuum similarly to the prior art(step 309). The substrates P1 and P2 which have been bonded together areconveyed to the post-irradiation unit 6, and ultraviolet light isirradiated onto the whole surface in a vacuum, thereby fully curing theadhesives B1 and B2 (step 310). In this case, ultraviolet light isirradiated from the side of the substrate P1 on which thesemi-transparent metal film has been sputtered. The disk D which iscompleted by curing of the adhesive is output from the output position35 (step 311).

Effects of Embodiments

According to the present embodiment described above, by making theadhesive B1 which is applied to the substrate P1 that is to be bondedthicker than the adhesive B2 which is applied to the substrate P2, andprovisionally curing the adhesive B1, then a partial region is cured inadvance, and the occurrence of exhaust gases is supplied when thesubstrates P1 and P2 are bonded together in a vacuum, and the occurrenceof residual air bubbles can be reduced. FIG. 4 shows the results ofreduction of air bubbles. From this, in the embodiment described above,the level of air bubbles when the film thickness ratio (B2/B1) of theadhesives B1 and B2 was varied indicates a greater air bubble reducingeffect, the greater the thickness of the adhesive B1 on the side whichis provisionally cured.

This is thought to be because there is a greater amount of materialwhich produces air bubbles in the thicker adhesive, but by carrying outprovisional curing of this adhesive, a greater reduction is achieved inthe amount of gas produced from inside the adhesive. The level of airbubbles was checked visually after bonding and completing full curing:level 0.0 indicates no bubbles, level 1.0 indicates the presence of somebubbles, level 2.0 indicates a moderate presence of bubbles and level3.0 indicates a large presence of bubbles. In the prior art example, inwhich adhesive was applied to the same thickness on both substrates andthe substrates were bonded together without provisional curing, thelevel was 3.0.

Furthermore, since one adhesive B1 is provisionally cured, then abeneficial effect in suppressing flowing movement is obtained, and hencein-circumference variation of the adhesive layer thickness after bondingtogether the substrates P1 and P2 can be suppressed. Moreover, theirradiation of ultraviolet light in order to achieve provisional curingis carried out when spin coating has finished and there is no scatteringof the adhesive B1, and consequently, none of the adhesive scatteredduring spin coating will have started to cure. Therefore, there is noneed to separate uncured adhesive from cured adhesive in the recoveredadhesive, and hence the recovered adhesive can be reused withoutproblem. Performing irradiation at a separate location to spin coatingalso provides similar beneficial effects. Since the spot irradiationwhich is carried out during rotation of the substrate P1 only involvesirradiation onto a limited partial region, then there is no problem inrecovering the adhesive.

Furthermore, the thickness of application can be adjusted easily byaltering the application amount of the adhesive, the conditions ofrotation, and the number of spreading operations. In particular, if itis wished to increase the application thickness, then a uniformthickness can be ensured by forming a cured portion and then applyingadhesive in a superimposed fashion while heating, as described above,and consequently the beneficial effect in suppressing in-circumferencevariation can be enhanced.

Further embodiments

The present invention is not limited to the embodiments described above.The extent of curing of the one adhesive prior to bonding is not limitedto that specified above. Consequently, even if the thicker adhesivewhich has been applied to one substrate is fully cured rather thanprovisionally cured, by means of various techniques such as irradiatingin a vacuum, irradiating after purging with an inert gas, increasing theirradiation intensity, increasing the irradiation time, or the like, itis still possible to obtain beneficial effects in suppressingin-circumference variation after bonding. In this case, adhesivenessduring bonding is ensured by the other adhesive which has not beencured.

Furthermore, by providing a first pre-irradiation unit 4 a whichirradiates ultraviolet light onto the substrate P1 and a secondpre-irradiation unit 4 b which irradiates ultraviolet light onto thesubstrate P2, as shown in FIG. 5, it is possible to bond the substratestogether after provisionally curing both of the adhesives B1 and B2. Bythis means, it is possible to suppress in-circumference variation yetfurther, as well as obtaining an effect in reducing the occurrence ofair bubbles.

Adhesive which is applied thinly becomes less liable to flow, thesmaller its thickness. For instance, if adhesive is applied in anextremely thin layer (of the order of several microns; a thicknesswhereby a cured portion is formed by spot irradiation), then flowingmovement is suppressed and a beneficial effect in suppressingin-circumference variation after bonding is obtained. Accordingly, ifthe adhesive that is not to be cured provisionally is applied in anextremely thin layer, then an increased effect in suppressingin-circumference variation may be achieved. A beneficial effect insuppressing in-circumference variation is also obtained if this thinlyapplied adhesive is provisionally cured.

Furthermore, the adhesive used is not limited to an ultraviolet-curableresin, and it is also possible to use various adhesives, such as resinswhich are used by other sources of electromagnetic radiation (includinglaser light), or thermally curable resins, or the like. Consequently,various different types of electromagnetic radiation can be irradiated,such as ultraviolet light, infrared light (including heat), laser lightof a prescribed wavelength, or the like, depending on the type of resinused. In the embodiment described above, in order to apply the adhesiveto a large thickness, a cured portion is formed and adhesive is thenapplied again in a superimposed fashion, but it is also possible toachieve a large thickness by simple superimposed application, or by asingle application operation in which the dropped volume of adhesive isincreased. It is also possible to omit heating for spreading theadhesive. Furthermore, bonding does not necessarily have to be carriedout in a vacuum.

There may be one or a plurality of application units for applyingadhesive. For example, it is possible to use a common spin coatingdevice for the first substrate and the second substrate. Superimposedapplication may be carried out using a plurality of spin coatingdevices. The application unit is not limited to a spin coating deviceand includes any current or future apparatus which can be used to applyan adhesive.

Furthermore, the pre-irradiation unit may be situated at any position,provided that it is after the spin coating step and before the bondingstep. For example, it may be disposed in the spin coating device, or inthe conveyance path from the spin coating device to the turntable. Itmay be provided for either one of the substrates P1 and P2 on theturntable, or for both of the substrates.

The size, shape and material, and the like, of the substrate can bechosen freely, and the present invention can be applied to any substratewhich may be used in the future. Consequently, as well as beingapplicable to disks for recording media of any format, it may of coursealso be applied to write-once read-many type recording media andrewriteable recording media. Furthermore, in addition to disks forrecording media, the present invention can also be applied to anysubstrates which are bonded together by means of adhesive. In otherwords, reference to “substrate” in the claims is not limited to acircular disk-shaped substrate, or the like, but rather is a broadconcept which includes flat plane-shaped products.

1. A bonding method for bonding a first substrate and a second substrateby means of an adhesive which undergoes curing by irradiation ofelectromagnetic radiation, the method comprising: applying the adhesive,at respectively different thicknesses, to one surface of the firstsubstrate and one surface of the second surface; irradiatingelectromagnetic radiation onto the thicker of the adhesive applied tothe first substrate and the adhesive applied to the second substrate;bonding together the surface of the first substrate to which theadhesive has been applied and the surface of the second substrate towhich the adhesive has been applied; and irradiating electromagneticradiation onto the adhesive between the first substrate and the secondsubstrate.
 2. A bonding method for bonding a first substrate and asecond substrate by means of an adhesive which undergoes curing byirradiation of electromagnetic radiation, the method comprising:applying the adhesive, at respectively different thicknesses, to onesurface of the first substrate and one surface of the second surface,irradiating electromagnetic radiation onto both the adhesive applied tothe first substrate and the adhesive applied to the second substrate;bonding together the surface of the first substrate to which theadhesive has been applied and the surface of the second substrate towhich the adhesive has been applied; and irradiating electromagneticradiation onto the adhesive between the first substrate and the secondsubstrate.
 3. A bonding apparatus for bonding together a first substrateand a second substrate by means of an adhesive which undergoes curing byirradiation of electromagnetic radiation, comprising: at least oneapplication unit for applying the adhesive, at respectively differentthicknesses, to one surface of the first substrate and one surface ofthe second substrate; a pre-irradiation unit for irradiatingelectromagnetic radiation onto the thicker of the adhesive applied tothe first substrate and the adhesive applied to the second substrate; abonding unit for bonding together the surface of the first substrate towhich the adhesive has been applied and the surface of the secondsubstrate to which the adhesive has been applied; and a post-irradiationunit for irradiating electromagnetic radiation onto the adhesive betweenthe first substrate and the second substrate.
 4. A bonding apparatus forbonding together a first substrate and a second substrate by means of anadhesive which undergoes curing by irradiation of electromagneticradiation, comprising: at least one application unit for applying theadhesive, at respectively different thicknesses, to one surface of thefirst substrate and one surface of the second substrate; apre-irradiation unit for irradiating electromagnetic radiation onto boththe adhesive applied to the first substrate and the adhesive applied tothe second substrate; a bonding unit for bonding together the surface ofthe first substrate to which the adhesive has been applied and thesurface of the second substrate to which the adhesive has been applied;and a post-irradiation unit for irradiating electromagnetic radiationonto the adhesive between the first substrate and the second substrate.5. The bonding apparatus according to claim 3, characterized in that theapplication unit has at least one spin coating device for spreading theadhesive by causing the first substrate and the second substrate torotate, the bonding apparatus further comprising control means forcontrolling the spin coating device such that conditions of rotation arerespectively different for the first substrate and the second substrate.6. The bonding apparatus according to claim 3, characterized in that theapplication unit comprises at least one spin coating device forspreading the adhesive by rotating the first substrate and the secondsubstrate, the bonding apparatus further comprising control means forcontrolling the spin coating device such that the number of spreadingoperations is respectively different for the first substrate and thesecond substrate.
 7. The bonding apparatus according to claim 4,characterized in that the application unit has at least one spin coatingdevice for spreading the adhesive by causing the first substrate and thesecond substrate to rotate, the bonding apparatus further comprisingcontrol means for controlling the spin coating device such thatconditions of rotation are respectively different for the firstsubstrate and the second substrate.
 8. The bonding apparatus accordingto claim 4, characterized in that the application unit comprises atleast one spin coating device for spreading the adhesive by rotating thefirst substrate and the second substrate, the bonding apparatus furthercomprising control means for controlling the spin coating device suchthat the number of spreading operations is respectively different forthe first substrate and the second substrate.